This invention relates to a substrate for ultra-small type thick film hybrid IC with a thick film microresistor and a process for preparing the same, a power source module for mobile radio communication using the same and a cellular radio communication system.
Heretofore, a hybrid IC has been prepared according to either thin film process or thick film process. According to the thin film process, a film is prepared by sputtering, etc. and conductor lines can be made much finer, for example, as fine as a resolution of 20 .mu.m, through a photolithography. However, even if the conductor lines can be much made finer, a resistor to be used together cannot be made much finer, because the resistivity of the resistor material to be usually used in the thin film process is very low.
Usually, a sheet resistance of 10 .OMEGA./.quadrature. to 1 M.OMEGA./.quadrature. is required for the hybrid IC. In order to make a resistor with a high sheet resistance such as about 100 k.OMEGA./58 to about 1 M.OMEGA./.quadrature. from a typical thin film resistor material such as TaN, etc., a larger area is required. For example, when calculation is made on the basis that the resistivity of TaN resistor material is 250.mu..OMEGA.-cm, the film thickness of the resistor is 1 .mu.m, and the line width of the resistor is 20 .mu.m, the length of the resistor will be 80 cm at 100 k.OMEGA./.quadrature. and 800 cm at 1 M.OMEGA./.quadrature.. When a resistor is designed at a resistor line width of 20 .mu.m, the resistor dimension will be 5 mm.times.5 mm at 100 k.OMEGA./.quadrature. and 16 mm.times.16 mm at 1 M.OMEGA./.quadrature.. Thus, even if the conductor lines can be made much finer according to the thin film process, the resistor cannot be made finer and it is difficult to make a ultra-small type, hybrid IC.
According to the thick film process, a film is formed usually by printing, as disclosed in Denshi Gijutsu, Vol. 25, No. 14, (1983), pages 10-11, 42-43, 78-79 and Japanese Patent Applications Kokai (Laid-Open) Nos. 53-133,501, 58-108,792, 52-137,667, 52-137,667, 53-65,970 and 59-201,482. However, there is a limit to making the conductor lines and the resistor much finer owing to the printing used for the preparation of a film. The limit thereto by the printing is now about 300 .mu.m. However, the reliability of the thick film resistor material is very high and the thick film process has been so far actually applied. Furthermore, the resistivity of the thick film resistor material is very high and a resistivity of 1,500 .OMEGA.-cm is possible (sheet resistance of 1 M.OMEGA./.quadrature. at a film thickness of 15 .mu.m). Thus, it seems that the thick film process is particularly suitable for making the resistor much smaller. However, as a result of various tests on combinations of copper conductor lines with thick film microresistors it has been found that it is difficult to control the resistance only by simple combinations of copper conductor lines with thick film microresistors.
That is, a resistor with a stable resistance can be obtained by forming copper conductor lines and thick film resistors according to a conventional thick film process only when the thick film resistors are long enough, but it has been found that, when the length of the thick film resistors is made smaller and smaller, the resistance will be much fluctuated and a stable resistance will be no more obtained. In this connection, the present inventors measured resistance of thick film microresistors to be combined with highly fine copper conductor lines by changing the length of the resistors to 100 .mu.m, 200 .mu.m, 300 .mu.m, 400 .mu.m, 500 .mu.m, 700 .mu.m or 1,000 .mu.m while keeping the width of the resistor constant at 300 .mu.m. The results of measurements are shown in FIG. 1 in terms of a ratio of sheet resistances (R.sub.x) of resistors having various lengths to the sheet resistance (R.sub.4.0) of a resistor having a size of 4 mm.times.4 mm, i.e. R.sub.x /R.sub.4.0. It is apparent therefrom that resistors having lengths of 500 .mu.m or more have a resistance substantially equal to the nominal sheet resistance of thick film resistor paste, and resistors having lengths of less than 500 .mu.m have only very lower sheet resistances. This phenomenon will be a very large obstacle to designing the resistance of a resistor.
In order to investigate causes for this phenomenon, the present inventors conducted a line analysis of the boundary surface between the highly fine copper conductor lines and the thick film microresistors by EPMA, and have found that the glass in the thick film microresistor and on the highly fine copper conductor line side is diffused into the highly fine copper conductor lines to make the resistivity of the resistor much lower, because the highly fine copper conductor lines and the thick film microresistor are both made from materials of electroconductive powders and glass.
It has been found from the foregoing results that it is difficult to obtain an ultra-small hybrid IC based on a combination of highly fine copper conductor lines with thick film microresistors unless the diffusion of glass from the thick film microresistors into the highly fine copper conductor lines is suppressed.